Semiconductor lasers are finding use in a variety of fields of technology. For instance, such lasers have found wide use in optical fiber communication systems. The operating speed of optical fiber communication systems has increased dramatically, to the point where at least one commercially available system operates in the Gigahertz range. The trend towards higher and higher operating speed is expected to continue. Thus it would be highly desirable to have available semiconductor lasers that allow simple high speed modulation of their output.
In addition to increasing information throughput through an increase in the bit rate at a single, predetermined wavelength, thought is also being given to increasing throughput through wavelength division multiplexing. For this application it would be desirable to have available semiconductor lasers that can operate at more than one wavelength, and in particular, that can be switched rapidly between two (and possibly more) wavelengths. Such lasers would also be of interest for application in optical information storage systems, especially if the two output wavelengths are relatively widely separated. For instance, the longer of the two wavelengths could be used to "read" stored information, and the shorter could be used to "write" information into the storage medium.
Lasers that emit at two wavelengths are known to the art. See, for instance, N. K. Dutta et al., Applied Physics Letters, Vol. 48 (25), pp. 1725-1726; H. E. Maes et al., Electronics Letters, Vol. 18(9), pp. 372-374; and Y. Tokuda et al., Applied Physics Letters, Vol. 51 (21), pp. 1664-1666. These publications disclose lasers comprising two separate and independent active regions, one for each wavelength. Such devices are difficult to manufacture and thus can be expected to be costly.
A particular kind of semiconductor laser known to the prior art is the so-called "quantum well" laser. See for instance W. T. Tsang et al., IEEE Journal of Quantum Electronics, Vol. QE-20 (10), pp. 119-1132 (incorporated herein by reference), wherein quantum well lasers (including single quantum well lasers) and their growth by molecular beam epitaxy are discussed. See also N. B. Patel et al., IEEE Journal of Quantum Electronics, Vol. QE-23 (6), pp. 988-992, which discusses threshold current considerations in graded barrier single quantum well lasers.
M. Mittelstein et al., Applied Physics Letters, Vol. 49(25), pp. 1689-1691, report that, by cleaving single quantum well lasers to progressively shorter lengths (spanning the length 70-470 .mu.m) it is possible to change the lasing wavelength. As is well known, the (modal) gain of a laser is a function of the length of the active region of the laser. Thus, cleaving otherwise identical lasers to shorter and shorter lengths results in lasers having lower and lower modal gain. This gain is conventionally measured in wave numbers (cm.sup.-1). Mittelstein et al. report an abrupt change in lasing wavelength at about 100 cm.sup.-1. This change was identified with the onset of lasing from the second sub-band of the quantum well.
Q-switching, well known in dye lasers, is also known in semiconductor lasers. See, for instance, D. Z. Tsang et al., Applied Physics Letters, Vol. 45 (3), pp. 204-206, which discloses a Q-switched laser comprising an amplifier section and, electrically substantially isolated therefrom, a modulator section. The length of the former is in the range 150-250 .mu.m, and of the latter in the range 25-75 .mu.m. A "waveguide" section between the amplifier and modulator sections is 25 .mu.m long. The gain of the laser cavity can be changed by changing the (reverse) electrical bias on the modulator section, making possible amplitude modulation of the laser output. The paper reports that devices have been continuously operated with full on/off modulation rates of 10 GHz.
Amplitude modulation by means of Q-switching of a multi-quantum well laser with an intra-cavity monolithic loss modulator has also been reported by Y. Arakawa et al., Applied Physics Letters, Vol. 48 (9), pp. 561-563. These workers too report on a semiconductor laser comprising an amplifier section and, electrically substantially isolated therefrom, a modulator section. The lengths of the amplifier and modulator sections were 250 and 50 .mu.m, respectively, and the resistance between the two sections was 5k.OMEGA.. Varying the electrical bias on the modulator section resulted in amplitude variation of the radiation emitted by the device.
Y. Tokuda et al., Applied Physics Letters, Vol. 49(24), pp. 1629-1631, have produced wavelength switching in a single quantum well laser by means of appropriate variation of the injection current. Since the change in the current involved is typically relatively large, it can be expected that the switching is attended by substantial chirp of the output radiation.
For reasons discussed at least in part above, it would be highly desirable to have available a semiconductor laser whose radiation output could be easily and rapidly modulated and/or switched between two predetermined wavelengths, especially relatively widely separated wavelengths. This application discloses such a laser. It also discloses a method of operating such a laser, and further discloses a laser amplifier having advantageous features.